PD-1 is recognized as an important player in immune regulation and the maintenance of peripheral tolerance. PD-1 is moderately expressed on naive T, B and NKT cells and up-regulated by T/B cell receptor signaling on lymphocytes, monocytes and myeloid cells (1). Two known ligands for PD-1, PD-L1 (B7-H1) and PD-L2 (B7-DC), are expressed in human cancers arising in various tissues, and PD-L1 expression has been associated with poor prognosis and reduced overall survival, irrespective of subsequent treatment, in studies on different cancers, e.g., ovarian, renal, colorectal, pancreatic, liver cancers and melanoma (2-12). Similarly, PD-1 expression on tumor infiltrating lymphocytes was found to mark dysfunctional T cells in breast cancer and melanoma (13-15) and to correlate with poor prognosis in renal cancer (16). Thus, it has been proposed that PD-L1 expressing tumor cells interact with PD-1 expressing T cells to attenuate T cell activation and evasion of immune surveillance, thereby contributing to an impaired immune response against the tumor.
Several monoclonal antibodies (mAbs) that inhibit the interaction between PD-1 and one or both of its ligands PD-L1 and PD-L2 are in clinical development for treating cancer. These include nivolumab and pembrolizumab, which bind to PD-1, and MPDL3280A, which binds to PD-L1 (17-19). While clinical studies with these PD-1 antagonists have produced durable anti-tumor responses in some cancer types, a significant number of patients failed to exhibit an anti-tumor response.
Tumor expression of PD-L1 has been investigated as a predictive biomarker to identify tumors that are likely to respond to anti-PD-1 blockade, and published studies have generally described immunohistochemistry (IHC) analysis of frozen or formalin-fixed, paraffin-embedded (FFPE) tumor tissue sections stained with a primary antibody that binds to membrane-bound PD-L1 (20-23).
Recently, an enzyme-linked immunosorbent assay (ELISA) was used to detect a soluble form of PD-L1 (sPD-L1) in human serum and in the culture supernatant of PD-L1 expressing cells (24). Because sPD-L1 levels in cell culture supernatants were lower in the presence of a matrix metalloproteinase inhibitor (MMPI), and correlated with the expression of membrane-bound PD-L1 (mPD-L1) on the cells, it was proposed that sPD-L1 is produced through proteolytic cleavage of mPD-L1. Also, since sPD-L1 has been shown to bind to PD-1, have immunosuppressive activity and is associated with aggressive renal cell carcinoma, it has been proposed that circulating sPD-L1 can contribute to tumor immune evasion (25). Thus, a need exists for sensitive and specific assays to detect sPD-L1 in human serum.